gianfar: Remove unused field grp_id from gfar_priv_grp

grp->grp_id is obsolete. It has no use in the current driver.
Remove it from gfar_priv_grp and put the 'rstat' member
in its place, in the 2nd cache line, as rstat needs fast access.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>